424 POPULAR LECTURES AND ADDRESSES. 



the other circumstances on which the length of 

 the spark might depend being left unmeasured. 

 By Robinson's and Coulomb's experiments was 

 established the law of electrostatic force, according 

 to which two small bodies, each electrified with 

 a constant quantity of electricity, exercise a mutual 

 force of attraction or repulsion, according as the 

 electricity is similar or dissimilar, and which varies 

 inversely as the square of the distance, when the 

 distance between the two bodies is varied. 



In physical science generally, measurement in- 

 volves one or other of two methods : a method 

 of adjustment to a zero, or what is called a null 

 method ; and a method of measuring some con- 

 tinuously varying quantity. This second branch 

 of measurement was illustrated in Coulomb's and 

 Robinson's experiments, where the law was de- 

 termined according to which the electric force 

 varies, when the distance between the mutually 

 influencing bodies varies continuously. The other 

 mode of experimenting in connection with measure- 

 ment is illustrated by another exceedingly import- 

 ant phenomenon, bearing upon electrical theory, 



